The present invention relates to the drilling of subterranean wells. Particularly, this invention relates to methods and equipment for acoustically looking ahead of the drill bit, while the well is being drilled, e.g., one-hundred meters or more, analyzing and interpreting the data to identify and predict formation characteristics and properties of geologic features and structures within the planned drill path before being penetrated, and in response thereto, optimizing drilling procedures and equipment. More specifically, the xe2x80x9clook aheadxe2x80x9d information may be utilized to cost effectively maximize rate of penetration (ROP) and mechanical efficiency of the drilling system through use of more effective and efficient drill bits and related equipment. The result is improved ROP as compared to use of less efficient bits or equipment which otherwise likely would have been used and reduced risk of incurring unanticipated drilling hazards.
When drilling subterranean wells, whether for hydrocarbons, water or other minerals, it is usually desirable to understand and obtain useful working knowledge of the formations, strata, environmental and hole conditions that may be encountered when drilling the wellbore. The more of this information that is available prior to actually encountering the change in formation or the drilling hazard, the more effectively the drilling plan may be designed and if needed, modified during drilling to provide a more cost effective response, efficiently drilled wellbore and safer drilling operation. The range of uncertainty in these conditions varies considerably from relatively low uncertainty in areas that have significant historical drilling data which may have been obtained from previous regional drilling experiences, to very high uncertainty in (a) exploratory wells, (b) wells in geologically complex formations, and (c) when drilling into previously unpenetrated horizons, such as when deepening a well. When drilling in areas of high uncertainty or risk the quality and quantity of useable information becomes more critical in optimizing the drilling program.
Techniques and equipment have been developed to gather and make available such information to improve drilling planning, efficiency and safety, including information gathered on both a macro and micro scale before the well is drilled and information gathered in real time, and while the well is being drilled. Room for improvement remains, however, in the quality, precision and timeliness of gathering and interpreting information so as to effect desirable improvements in a drilling program. Advances in measurement while drilling (MWD) equipment and seismic techniques are improving drilling efficiency and well quality, however, limitations remain which often result in economic and mechanical losses in drilling efficiency. Traditionally the vast majority of the information used in planning and drilling a well is obtained and analyzed prior to drilling. Once drilling has begun, relatively few changes are made in the drilling plan because the original plan is typically conservatively over-designed to account for reasonably anticipated but still unidentified drilling hazards, usually resulting in less than optimum ROP. One reason for this is that while drilling a well, relatively little additional information is gathered which may be useful in timely predicting unexpected changes in environment or formation characteristics before the changes are actually encountered. Unnecessarily accelerated bit wear or premature bit destruction may be avoided under prior art by employment of more mechanically conservative, less aggressive cutting bits.
Under the prior art it is difficult if not impossible to identify unexpected or previously unforeseen changes in geology, structure, stratigraphy, pore-pressure, rock matrix, faults, formation consolidation or other environmental alterations or hazards that the drill bit may encounter, with sufficient lead time before these events are actually experienced so as to timely implement prudent changes in the drilling program, including changing the bit selection. The inability to timely identify the presence and location of these formation changes may prevent improvement modifications in the drilling program, e.g., modifying bit selection, drill string and downhole assembly design, weight on bit, rotational speed (rpm) and determination of whether to rotate the drill string or xe2x80x9cslidexe2x80x9d drill utilizing a downhole motor. Failure to implement these and/or other modifications to the drilling program parameters often results in increased drilling time and costs through decreased ROP, a lower quality wellbore, less than optimal control of formation damage and decreased safety while drilling.
The prior art is incapable of fully effecting the desired improvements in optimizing the drilling program. The prior art is deficient in timely, precisely and confidently determining the numerous formation rock and pore properties which exist ahead of the bit in order to avoid excessive over-design of the drilling program. For example, it is generally accepted that in soft to medium-hard formations, polycrystalline diamond cutter (PDC) bits usually yield the best ROP. These bits are also among the more expensive. If an unexpected hard stringer or formation is encountered by a PDC bit, the PDC bit may be quickly destroyed or incur accelerated wear and damage. Thus, an improperly designed component in the drilling program, such as an improperly selected bit can result in excessive delays and costs, including rig time to pull the pipe out of the hole (trip), change bits and run the pipe back to bottom. Pieces of the destroyed bit may also have to be xe2x80x9cfishedxe2x80x9d out of the hole before drilling may be continued, thereby resulting in additional xe2x80x9ctripsxe2x80x9d and significant costs.
An additional important parameter in controlling well costs and well quality is pore pressure determination. Identification of the presence of over-pressured or under-pressured zones should be made before they are encountered by the bit. Over-pressured zones can result in loss of well control, potentially leading to loss of the well, the drill-string, the drilling rig and possibly human life. Under-pressured zones can result in loss of costly drilling fluid, formation damage, loss of well control, stuck pipe and loss of the drill string or the well.
Proper planning is important in avoiding drilling hazards in the most cost-effective manner and in maximizing drilling ROP efficiency. Proper planning, however, requires timely, sufficiently detailed, useable information. Historically, an over-designed drilling plan and inflated equipment safety factors are typically built into a well plan to mitigate the effects of unforeseen hazards, often resulting in excessive and usually significant additional costs. Timely, useable information is valuable in optimizing a drilling program and the advantages may be reflected throughout the many costs included in drilling a well.
Techniques are known which use acoustic signals to gather and process data while drilling that relate to either recently drilled formations or undrilled formations within close proximity of the drill bit or to information pertaining to the drill string and downhole conditions thereof. One common technique is to gather three-dimensional seismic data at the surface or at sea before drilling the well in an attempt to map and identify relatively significant subsurface features, as disclosed for example in U.S. Pat. No. 5,555,531. This method may also include preparing high resolution three-dimensional vertical profiles from the data, preparing an artificially-illuminated and rendered surface based on the data, potentially identifying significant reflective sub-surface features, including those which may be hazardous. However, with increasing depth of investigation these seismic methods may suffer decreasing resolution and cumulatively increasing error. In addition, although seismic while drilling techniques are known, seismic is typically not utilized once drilling begins.
A known method of obtaining formation information while drilling is using seismic pulse generators. Seismic pulse generators have been used to obtain data on formations at relatively the same depth as the drill bit and also to look ahead of the drill bit as seen in U.S. Pat. No. 4,207,619. That patent discloses the use of numerous sensors arranged in symmetrical array on the surface, thereby resulting in significant additional expense, complexity and time to deploy the sensors, retrieve and then process the data. This can be additionally problematic in offshore drilling, where there is often a great distance between the surface of the water and ocean floor.
Another system using an acoustic transducer in drilling applications is disclosed in U.S. Pat. No. 5,798,488. That system preferably utilizes an acoustic transducer system which is integrated into the MWD tools of the drill string and generates acoustic signals in the rock ahead of the bit by vibrating the drill bit and the attached masses on the lower end of the drill string. The patent discloses an embodiment where the acoustic transducer is made as an integral part of the bit. In either embodiment, drilling is paused and the acoustic transducer may be electrically vibrated, thus propagating acoustic waves through the drill bit and into the formation ahead of the bit. The reflected acoustic signals may be collected by the same transducer that generated the signal. U.S. Pat. No. 5,798,488 discloses that it is desirable when drilling for oil to know what strata may lie ahead of the bit in order to allow the most appropriate drilling parameters to be employed, to know the location of the bit relative to anticipated or known rock features and to identify over-pressure and under-pressure regions. It fails, however, to disclose a method to enhance rate of penetration and extend bit life.
In other prior art commonly known as MWD or LWD (logging while drilling), instruments are known in which may be positioned within the drill string as part of a bottom hole assembly and which may generate and record acoustic signals. Measurements may be made either while drilling or during brief intermissions in drilling and may provide useful information in evaluating drilling and formation characteristics such as bit location and direction, hole deviation and weight on bit. These measurements may facilitate evaluation of various formation and rock properties such as bed boundaries, porosity, resistivity and natural radioactivity measurements. MWD and LWD have proven to be useful for improving the quantity and quality of information pertaining to environmental properties or operating parameters in or around the vicinity near the wellbore and are generally limited to formations which have been penetrated by the bit. As measurements are made, the MWD or LWD system typically transmits the data in real-time to the surface or may store the data for later transmittal to the surface, or the system may process the data downhole and then transmit a processed signal. The scope of investigation and measurement with these types of instruments or tools is typically limited to inside or near well-bore evaluation. Most often, MWD or LWD measurements may be obtained for a formation or geologic feature after that formation or feature has been penetrated by the bit, or where the geologic feature is located within close proximity to the bit or measuring tools in drill string.
This prior art, however, fails to provide for drilling program improvements, which may safely increase the rate of penetration and bit life. Applications are desired that investigate formation characteristics sufficiently far ahead of the bit such that drilling hazards may be predicted and avoided and drilling opportunities identified and exploited, such that ROP may be increased by selecting the optimum bit and refining the related drilling parameters in order to avoid excessive over-designing.
The present invention provides a method for improving ROP and drilling efficiency by acoustically investigating or looking ahead of the drill bit to detect and analyze geologic features ahead of the bit which may be qualified as drilling hazards. A drilling hazard, as the term is used herein, may not necessarily be dangerous or inherently destructive to drilling equipment but rather may be a geologic or environmental feature which could potentially impose detrimental effects upon drilling system performance, hole quality or drilling equipment, including those effects which may be inherently destructive. The methods of this invention utilize computer interpretation of acoustic information obtained by acoustic, electric and/or mechanical instruments and tools which may be integrated partially or wholly into the bottom hole assembly of the drill string and which may also be partially located at the surface. A technique for transmitting data or information from downhole to the surface may also be included. This invention pertains generally to methods for application or use of the calculated xe2x80x9clook aheadxe2x80x9d information to improve the decision making process with regard to implementing changes in the bit, downhole assembly or related drilling parameters to timely and prudently respond to identified potential hazards such that the ROP for the well may be improved as compared to the ROP which would be possible without the ability to look ahead of the bit.
It is an objective of this invention to provide a method that uses an acoustic system to locate, analyze and display the type of hazard, range and relative direction from the drill bit to the hazard. The acoustic system utilized by the methods of this invention may provide an acoustic xe2x80x9clook ahead of the bitxe2x80x9d and predict drilling hazards at least one-hundred (100) meters ahead of the bit with a resolution capability to identify and project hazards having a thickness of at least one (1) meter in thickness. The hazards may be detected as variations in acoustic impedance. The acoustic system should propagate acoustic signals and detect formations or hazards with strong acoustic contrasts, such as hard stringers, having a thickness of at least one (1) meter relative to less hard adjacent shoulder beds, thereby facilitating use of the methods of this invention. Other hazards may also be identified which may detrimentally effect ROP, including extremely soft or gummy formations such as xe2x80x9cgumbosxe2x80x9d or coal beds, such that bit selection, hydraulics, mud properties or other drilling parameters maybe appropriately adjusted to accommodate the existing bit before bit balling and pipe sticking or other ROP hindering problems may be encountered.
The methods of this invention may utilize an acoustic tool and related equipment, which propagates an acoustic wave at frequencies in the neighborhood of a few kilohertz. Compression and shear waves may both be propagated and evaluated. Acoustic wave properties in geologic formations are functions of numerous variables including porosity, rock matrix composition, overburden stress, pore pressure, temperature, fluid properties and grain texture. The acoustic waves may be slowed, distorted, elastically absorbed and reflected as they encounter variations in acoustic impedance, which may contrast with the impedance of preceding formations. The reflected signal may be received by some acoustic receiver arrangement and then transmitted to the surface for analysis or analyzed downhole, to project an acoustic visualization of the formation and geologic characteristics that may exist within the zone of acoustic investigation ahead of the bit. It is a feature of this invention to preferably use an acoustic system that combines the transmitter and receiver at a generally common location and eliminate the need to place receivers in multiple locations along the drill string.
The methods of this invention use the ability to project drilling hazards as a means to add net value to well economics by improving the ROP from two perspectives. First, it may permit adjustment of drilling equipment and parameters while the well is drilling. Second, this invention may permit application of more aggressive and efficiently designed drilling equipment and parameters in the planning stages of the well through reduced over-design due to unidentified hazards. Benefits from both perspectives may combine to improve the ROP.
The ROP improvements sought are xe2x80x9cnetxe2x80x9d ROP improvements, meaning improved drilling time over the entire course in which the look ahead system may be employed. It is possible that an alteration in a drilling parameter over a specific interval, in response to a detected hazard, may result in a temporary decrease in ROP as compared to the ROP which may have been obtained over that same interval using prior art equipment. For example, in utilizing a relatively expensive PDC bit, when a detected hard stringer is to be encountered, in lieu of changing to a hard rock roller cone button bit, WOB and RPM may be reduced such that the PDC bit may slowly cut that section carefully and deliberately in order that the PDC bit is not unacceptably damaged. However, over the course of the entire section drilled by the PDC bit, the overall ROP may be greater than the ROP that may have been achieved using prior art methods. With the methods of this invention, unanticipated hard stringers such as cherts, hard sandstones or hard dolomites may be timely identified, facilitating prudent bit changes or drilling parameter changes so as to accommodate the use of more efficient PDC bits and less over-designed downhole equipment in order to achieve an improved net ROP.
This improved drilling practice may be highly advantageous in drilling exploratory wells. An advantage of this invention over prior art methods is that, as discussed above, it may more readily facilitate use of PDC bits in lieu of the previously favored roller cone bits in exploratory and other wells which may have complex or uncertain formation properties, thereby improving the ROP. In addition, to facilitate improved ROP this invention may improve the selection of any other type or grade of more efficient bit or mill, including PDC, roller cone, blade, reamer, under-reamer or other cutter, than otherwise may have been selected without the benefit of acoustically looking ahead of the bit to identify hazards. Other bit options for enhancement may include bit tooth, button or cutter material, shape and hardness. This invention may thus accommodate the prudent design and/or selection of a more aggressive, faster penetrating bit for drilling the majority of the hole through utilization of acoustic projection technology to appropriately assess the nature of the identified drilling hazards. In addition, timely adjustments to other drilling parameters may also be made to accommodate a given bit and drilling assembly. For example, in order to accommodate use of PDC bits for drilling in areas of increased geologic uncertainty, drilling parameters such as the revolutions per minute (RPM), weight on bit (WOB) and downhole assembly design may be adjusted periodically while drilling. The result should be improved ROP, hole quality and drilling efficiency.
Identification of the existence, location and relative size of hazards such as hard streaks, coal seams, faults, salt sections, over pressure zones, gumbos and swelling clays may be beneficially identified and evaluated before they are encountered by a bit in order to safely and efficiently maximize ROP. For example, acoustic visualization of hazardous features which may lie ahead of the bit within a planned well path may also facilitate changing from an aggressive PDC bit to a less efficient hard-rock roller cone bit when encountering hard stringers. In the event a bit change is made, related drilling parameters may also be modified to accommodate the new bit type and revised drilling plan.
As stated previously, in addition to the methods of this invention facilitating ROP improvements while the well is drilling, consideration in the planning stages for use of the methods of this invention may also increase ROP, which may beneficially impact a drilling program such as enhancing rig scheduling and tool usage through reduced trips, number of bits and rig time required to drill the well. Other related enhancements from this invention which may be employed in the planning stages of the well to improve ROP include designing the size, amount and type of downhole equipment such as drill collars, mills and/or reamers, the necessity for and sizing of downhole motors, stabilizers, jars, rotary torque requirements and anticipated rotary speed.
A feature of this invention is the ability to increase bit life, thus minimizing the required number of bits required to drill a given hole section and also reducing the number of trips which must be made out of the hole to change to a new or different bit. The net effect is to increase ROP by reducing premature wear of the bit and reducing non-drilling rig time.
An additional feature is a reduction or prevention of occurrences of premature bit failures. Bit failures may often result in xe2x80x9cfishingxe2x80x9d operations to remove bearings or other debris from the hole before a new bit may be run. Bit failures result in increased bit costs and increased trip time to change the bit. Reduction or elimination of bit failures may also result in increased ROP through timely prediction of drilling hazards facilitating adjustments in drilling equipment or parameters so as to avoid destruction or accelerated wear of the bit.
Cumulative incremental benefits which impact ROP may thus be obtained from refinement of drilling parameters through use of this invention in both the pre-drill planning stages of the drilling program as well as the timely on-site detection and evaluation of drilling hazards identified while drilling such that the result is an improved net ROP as compared to the net ROP that may have otherwise been possible without the invention. The combined effect of these improvements upon ROP should result in enhanced economic and mechanical drilling efficiency. These and further objects, features and advantages of the present invention will become apparent from the following detailed description, wherein reference is made to figures in the accompanying drawings.